This invention relates to a variable capacity turbo supercharger, which utilizes for the variable capacity the character of the engine exhaust gas as gaseous fluid without using any movable part at all.
A turbo supercharger which is adopted as automotive supercharger engine is required to have a broad working range and also satisfactory supercharge pressure response at the time of sudden acceleration.
To meet the above demands, there has been a well-known variable capacity turbo supercharger with variable static vanes provided in a turbine nozzle. This turbo supercharger, however, has not yet been adopted for general passenger cars because of the facts that it greatly increases the number of components made of heat-resistant alloys which are expensive and difficulty capable of cutting, that it requires high accuracies of finishing and assembling, and that it requires a complicated link mechanism for harmonious operation of a plurality of static vanes, these requirements leading to a great cost increase.
As the variable turbo supercharger the above variable static vane type is ideal. Up to date, there has been realized two simplified variable capacity turbo superchargers.
One of these variable capacity turbo superchargers has a movable flap which is provided in lieu of turbine scroll tongues. The flap is formed at one end with a throat and are rotated about the opposite end to vary the area of the throat so as to attain variable capacity.
The other variable capacity turbo supercharger has a turbine scroll chamber which is dividable in the axial direction into two halves. An on-off valve is provided on an exhaust gas supply passage leading to one of the two scroll chamber halves and is adapted to change the throat area in two stages, thereby attaining the variable capacity.
In comparison to the aforesaid variable vane type, the variable flap type consists of a single flap. The scroll type axially dividable into two halves uses a simple on-off valve. In these types, therefore, the cost increase can be reduced. However, these types also require an actuator for driving the variable part and also a control unit for controlling the operation of the actuator. For this reason, a great cost increase compared to the conventional fixed capacity type is inevitable. Therefore, even if the simplified types noted above are adopted for ceramic turbo superchargers and twin turbos, further cost increase is inevitable, and the adaptation of these simplified types is impractical.
The supercharge pressure response is the most important performance of the current automotive turbo supercharger. In a variable capacity turbo supercharger in which the turbine throat area is varied as noted above, when suddenly accelerating the engine from the idling or the like operating condition in which the turbine rotor is rotated at a very low speed, the supercharge pressure response can be improved by temporarily extremely reducing the throat area and then continuously increasing the throat area. To this end, however, high response and displacement accuracy are required for the variable part drive unit, the control system is complicated, and the reliability is reduced.
At present, however, no continuous control corresponding to the situation of a sudden acceleration or like other than the constant situation has been realized. The most ideal variable static vane is not adopted for passenger cars, in which the acceleration property is imporant. This is so because even if the expensive variable vane type is adopted, the continuous control as noted above can not be realized.
Further, in the axially dividable scroll type as noted above, even if the on-off valve opening is continuously controlled in the manner as noted before, the throat area of the scroll chamber half without the on-off valve can not be extremely minimized out of the consideration for the entire engine-operating condition ranges. Besides, the scroll chamber half with the on-off valve can serve only as a bypass passage for controlling the gas velocity in the other scroll chamber half. Therefore, the gas having passed through this scroll chamber half does not provide an effective action.
Moreover, it generates undesirable vortex between adjacent turbine rotor vanes due to the speed difference between the gas from one scroll chamber half and the gas from the other scroll chamber half. For the above reasons, this type has no particular advantage.
Usually, therefore, in this type the on-off valve is switched to either fully open or fully closed state, but the response with respect to the supercharge pressure at the time of a sudden acceleration is no so satisfactory.